The exploration of natural resources requires many different technologies, expertise and large high-risk capital investments to be successful. Geophysical exploration involves the search for deposits by measuring the physical properties of rocks, such as remnant magnetization, electrical conductivity, radioactivity and the behavior during seismic shocks. Gravity methods, such as gravity gradiometry and gravimetry, use variations in the gravitational field above the Earth's surface to infer variations in the subsurface density.
Geochemical exploration involves the study of varying elemental concentrations in natural features such as plants, soil, lakes, streams, swamps and gases. Leaching and weathering disperses the components of resource deposits into the surrounding water, soil, vegetation and air to create chemically enriched zones known as geochemical anomalies.
Geomagnetic exploration involves the study of variations in the Earth's magnetic field above the Earth's surface to infer variations in the subsurface metallic content, which may create zones of magnetic anomalies in the spatial distribution of the Earth's magnetic field. To measure such magnetic anomalies, a magnetic anomaly detector is typically employed. One common magnetic anomaly detector comprises an optically pumped cesium vapor magnetometer, which works in tandem with a triaxial magnetometer providing data necessary for precise mechanical guidance of the cesium head of the vapor magnetometer. This magnetic anomaly detector is typically placed on an aircraft and used during airborne surveys. An exemplary optically pumped cesium vapor magnetometer is set out in the AN/ASQ-508(V) System Description published by CAE, Inc., Canada, Mar. 22, 2002.
Solid-state magnetic anomaly detectors based on the giant magnetoimpedance (GMI) effect in ultra-soft magnetic conductors have also been considered. These magnetic anomaly detectors are advantageous in that they are simple, robust and inexpensive as compared to optically pumped cesium vapor magnetometers. The sensing element in such magnetic anomaly detectors is, in general, a wire made of an amorphous magnetic alloy with high magnetic permeability, saturation magnetization and electrical resistivity, and very low coercivity and transverse anisotropy field. The impedance of the magnetic wire changes significantly with the strength of an applied static magnetic field, which modifies the circumferential permeability of the magnetic wire and thus, its circumferential magnetization process. Exemplary GMI-based magnetic anomaly detectors that use internal biasing obtained through torsional or tensile stress applied to the magnetic wire and that are driven by AC currents in the 10 to 100 MHz range of frequency, include those taught in U.S. Pat. No. 5,994,899, issued on Nov. 30, 1999 to K. Mohri and U.S. Pat. No. 6,727,692, issued on Apr. 27, 2004 to P. Ciureanu et al. Although these GMI-based magnetic anomaly detectors provide some advantages, they are less accurate than optically pumped cesium vapor magnetometers. As will be appreciated, a GMI-based magnetic anomaly detector capable of detecting a magnetic anomaly with a spectral noise density comparable to optically pumped cesium vapor magnetometers (i.e. about 10−8 Oe/√Hz) is desired.
It is therefore an object of the present invention to provide a novel magnetic anomaly detector and method.